The use of colloidal quantum dots (CQD) in optoelectronic devices requests both the fine control of their optical and transport properties. Transport in a CQD film is a multiscale process where hopping process occur at the nanoparticle scale and film morphology (cracks . . . ) is playing a role at the micrometric scale. Consequently not only the inter-particle tunnel barrier needs to be tune to adjust the coupling but a good long scale ordering is also requested. Atomic-ligand passivation (such as S2−, SCN− or Cl− and metal chalcogenides ligands) do address the shortening and lowering of the inter-particle tunnel barrier but they generally request polar solvent which come at the price of a more limited range of method to build the nanoparticle film. With such passivation the film remains strongly disordered. In disordered film the photo-activated carrier still need to perform a random walk to reach the electrodes which typically included hundred to thousand steps. To avoid this inefficient transport process several strategies have been developed among which the realization of QD-graphene hybrid to uncouple the absorption from the transport process or the use of nanogap.
With a nanometer long channel, capable of accommodating nanoparticles, the nanoparticle can be directly connected to the electrodes which avoid the post absorption diffusion transport of the carrier and its trapping. Moreover the short transport length reduced the transit time which tend to increase the photoconductive gain of the device. To realize these nanogaps several methods have been proposed including e-beam lithography, self-alignment method, electromigration or shadowing methods. In spite of this interest quantum dots remain tricky to connect to the electrodes and a poor overlap is obtained while using a spherical particle which size is of the same order of magnitude of the gap size.
One of the object of the present invention is thus to use nanoplatelets for connecting nanogap electrodes.
Motivation for nanogap based photodetector is first the increase of the gain. In a photodetector the responsivity, i.e. the hability of the active material to convert the light photon flux into a current expressed in A·W−1; is proportional to the product of the internal quantum efficiency by the gain R π ηg. The gain is itself the ratio of the photo carrier lifetime τ divided by the transit time τtransit, where the transit time is the time for a photogenerated charge to reach the electrode:
  g  =            τ              τ        transit              .  The internal quantum efficiency is the ratio of the number of charger carriers collected by the electronic device to the number of photons absorbed by the active material. The smallest the spacing between the electrodes the shortest the time for the carrier to reach the electrodes. As a consequence reducing the electrodes spacing from a few micrometers to a few nanometers potentially increases the gain by a factor 1000.
Other motivation for nanogap based photodetector is the fact that the volume reduction of the nanoparticle makes that it is easier to get rid of the defect of the film morphology. Indeed for micrometer scale film is common to observe crack formation into the film. These cracks in particular tend to be formed when a ligand exchange procedure on film is processed.
Finally another attractive aspect for nanogap based photodetector is the fact that transport is no longer driven by hopping. Consequently the noise level is not as high as the one associated with hopping transport.
Consequently the use of nanoplatelets for connecting nanogap electrodes could lead to outstanding properties, such as responsitivity and/or specific detectivity, which have not been reported until now in the prior art.